The process of electroerosion machining, now widely in use in industry, may be applied in various forms. For example, in "traveling-wire" electroerosion, a thin, continuous wire, tape or ribbon is employed as the tool electrode to form contours of various shapes in workpieces. In "sinking" electroerosion, a formed solid electrode may serve as the tool electrode to shape a cavity complementary in shape thereto in a workpiece. In "generic" electroerosion, a "non-formed" or generic electrode is constituted as the tool electrode and moves in a scanning manner relative to the workpiece to form an intricate cavity or contour therein.
In the varying practical forms of electroerosion, it is a common requirement that the erosive current be furnished in a succession of discrete, time-spaced current pulses, which can be produced by pulsing a DC output. It has been found to be important that the current pulses be square or rectangular in waveform, this being particularly the case where the workpiece is to be "rough" machined. A "roughing" operation requires long current pulses whose duration or "on" time .tau.on ranges, say, in excess of 50 microseconds and often in the millisecond order. Given a particular mode of operation, it is important that "square" current pulses have a particular duration .tau.on, a particular pulse interval or "off" time .tau.off and a particular current level Ip established in a particular combination to yield a particular set of machining results (e.g. surface roughness, overcut, relative electrode wear and stock removal) desired for that particular operation. Thus, the need arises to be able to obtain any "square" roughing-mode pulse with the pulse parameters .tau.on, .tau.off and Ip independently adjusted. This capability is indeed vital to a modern electroerosion generator as it is designed to apply to a variety of rough and relatively rough machining operations.
In the finish machining range, however, it has been found that different considerations must apply. The finer the surface finish sought, the shorter must the pulse duration be, towards and desirably into the nanosecond order. However with the shortening of such pulses, the difficulty or retaining the squareness of the erosive current pulses increases. Thus, when the pulse duration .tau.on must be as short as 10 microseconds or less, the pulse can be at least sinsoidal in its current-time characteristic, even when a conventional, most-advanced fine-pulsing erosion generator, either of switching or capacitor type, is employed. It should be noted that a typical switching-type erosion generator primarily for roughing modes may even fail to assure against the possibility that because of the short pulse duration the current may cease without rising to a desired level. At any event, while a short delay time which entails in the transitions between the "on" and "off" states of the switch is negligible in a longer, roughing pulse, it can no longer be so with any narrow, finishing pulse. While the capacitor-type pulsing system has been preferred because of its ability to secure a highly elevated current rise and also to reduce or eliminate the switching transitions, it is recognized that a capacitor discharge current is intrinsically sinusoidal, and its pulse duration and peak current must be more or less mutually dependent. These restrictions are also inherent to the modern switching-type system (devoid of a storage capacitor) in which a higher DC output is required to reduce the switching transitions as will be readily recognized. Thus, in the finish machining range, there have been severe limitations in forming erosive pulses, which do not arise in the rough machining range.
With respect to these limitations, I have discovered that problems arise in the actual performance of erosive machining pulses in the finish range which necessitates a pulse duration as short as 10 microseconds or less. There is a constant desire to improve the machining efficiency without affecting other machining factors, say, the surface finish. The breakage of a wire electrode in a traveling-wire erosion operation is fatal since the machining operation must be interrupted. The machining efficiency can be measured by the removal rate which is a function of the stock removal and the machining estability. The problem of wire breakage is associated with the electrode wear and the discharge stability. It has been found that the foregoing limitations inherent in the prior art have severely hindered attempts to achieve improved results in the finish erosion range.